Device-to-device (D2D) communication is direct communication between devices and is a new topic in 3GPP (3rd Generation Partnership Project) LTE (Long Term Evolution) Release 12. D2D communication could happen with wireless network coverage (e.g. for commercial case) or without network coverage (e.g. for public safety). FIG. 1 illustrates exemplary D2D communications with and without wireless network coverage. On the left side of FIG. 1, UE 101 and UE 102 are within the wireless network coverage of eNB (eNode B) 103, but they are communicating with each other directly (i.e. not through eNB 103). On the right side of FIG. 1, UE 104 and UE 105 are not within any wireless network coverage, and they are communicating with each other directly.
3GPP RAN1#76 meeting agreed eNB scheduling based resource allocation (Mode 1) as baseline method in network-coverage (INC) scenario and UE selection on its own based resource allocation (Mode 2) is baseline method in edge-of-coverage or out-of-coverage (OOC) scenario. In case of INC scenario, the resource collision could be avoided by eNB and the resource utilization is relatively high. However, in case of OOC scenario, the collision may not be avoided based on Mode 2 resource allocation method. If the collision probability is large due to congested load, it would impact the system performance very much.